Cathode ray tubes require numerous electrical potentials to be applied to the electrodes of the tube. One of these, the second anode has a very high potential, in the order of 30 kV or more in the case of color picture tubes, applied thereto. This potential is conventionally applied through the funnel by a connector hermetically sealed therein. This connector is conventionally called a second anode button. It is a substantially hollow, electrically conductive button having a thermal coefficient of expansion that substantially matches that of the glass into which it is sealed. Prior to insertion the button is oxidized so that an oxide to oxide bond is formed with the glass.
Prior buttons have been made from a 42%Ni-6%Cr iron alloy (ASTM F31) and more recently from a 47%Ni-6%Cr iron alloy (known commercially as N485). Other iron-based alloys that have been used for glass-to-metal seals include chromium-free nickel alloys (ASTM F30) and 18 to 26% chromium-iron alloys; however, neither of the latter two are known to have been used for anode buttons.
The thermal expansion-contraction differences between glass and metal generate the mismatch stresses that account for much of the breakage that takes place during heat cycles at fabrication. Examination of volume-temperature diagrams for the typical lead or soda-lime glasses used for the funnels show that they exhibit a non-linear behavior that can be matched to the volume-temperature curves of nickel-iron alloys in the 40 to 50% nickel range. However, due to their nickel content, these alloys are significantly more expensive than nickel-free alloys.